The present invention relates to a liquid crystal display device, more specifically, to a liquid crystal display device for performing a bright display using a reflection-type polarizing film by its reflection characteristic, or performing a display using a selective-reflection type film by its difference in color tones.
Recently, a reflection-type liquid crystal display device for performing a display by an external light source has been developed as a liquid crystal display device for portable information processing devices, and improvement of brightness and multi-color display has progressed. As a method for improving brightness, a method is tried in which a reflection-type polarizing film is provided on the side of a liquid crystal cell opposite to the visible side, and a printed layer is formed on the rear face of the reflection-type polarizing film. The reflection-type polarizing film has a transmission axis and a reflection axis as two optical axes orthogonal to each other and has characteristics for transmitting a light linearly polarized in the direction parallel to the transmission axis but for reflecting an incident light linearly polarized in the direction parallel to the reflection axis.
Moreover, as another method for improving brightness of the liquid crystal display device, a method using a selective-reflection characteristic of a cholesteric liquid crystal polymer is also considered.
The polarizing film conventionally and widely used in a liquid crystal display panel is an absorption-type polarizing film. The absorption-type polarizing film has two optical axes, a transmission axis and an absorption axis, orthogonal to each other, and it has characteristics for transmitting a light linearly polarized in the direction parallel to the transmission axis but for absorbing an incident light linearly polarized in the direction parallel to the absorption axis.
Therefore, the absorption-type polarizing film is used in combination with the aforesaid reflection-type polarizing film, whereby the liquid crystal display device has a large transmission characteristic when the transmission axes of the polarizing films are parallel to each other and a large reflection characteristic when the transmission axes are orthogonal to each other.
Accordingly, the liquid crystal display device has a large absorption characteristic (a black display) when the two absorption-type polarizing films are disposed in such a manner that the transmission axes thereof are orthogonal to each other, different from a transmission characteristic when they are disposed in such a manner that the transmission axes are parallel to each other.
In the case of the liquid crystal display device using two absorption-type polarizing films, a reflector is disposed on the rear face side of the absorption-type polarizing film disposed on the side opposite to the visible side in relation to the liquid crystal cell, whereby a bright display is performed by reflecting incident light from the external light source to the visible side in a transmission state and a dark display is performed in an absorption state. In this case, however, since the reflected light which is viewed passes through the absorption-type polarizing film positioned on the reflector twice, the light is absorbed partly, resulting in a display of which brightness somewhat decreases.
Moreover, the reflector having light scattering properties is used, absorption by the absorption-type polarizing film occurs due to instability in polarization by the reflector, whereby brightness be come impaired.
Furthermore, in a dark environment without an external light source, since the visibility of the display of the liquid crystal display device extremely deteriorates, an auxiliary light source is provided in the liquid crystal display device in many cases. In that case, a reflector of a transflective-type is used in place of the reflector without a transmission characteristic.
In this case, the bright display by reflection of an incident light from the external light source, briefly explained except for the liquid crystal layer, corresponds to the case where the transmission axes of the two absorption-type polarizing films are parallel to each other, resulting in a bright display also in the case where the auxiliary light source is used. The converse dark display corresponds to the case where the transmission axes of the two absorption-type polarizing films are orthogonal to each other, resulting in a bright display also in the case where the auxiliary light source is used.
In contrast to the above, according to the liquid crystal display device using an absorption-type polarizing film and a reflection-type polarizing film in combination, in the case where the external light source is used, a bright state is a state where the transmission axis of the absorption-type polarizing film and the reflection axis of the reflection-type polarizing film are parallel to each other to obtain a reflection characteristic of the reflection-type polarizing film. Accordingly, the reflection-type polarizing film itself reflects incident light resulting in a bright display. Conversely, a dark state is a state where the transmission axis of the absorption-type polarizing film and the transmission axis of the reflection-type polarizing film are parallel to each other and uses the transmission characteristic, and thus it is required to dispose or print a light absorbing material on the rear face of the reflection-type polarizing film. As above, bright and dark displays with excellent contrast can be performed by means of the liquid crystal display device using the absorption-type polarizing film and the reflection-type polarizing film.
The structure of the conventional liquid crystal display panel of the liquid crystal display device as above is explained with reference to the drawings. FIG. 16 is a plane view showing a plane structure of the principal portion thereof, and FIG. 17 is a partly enlarged sectional view along the Axe2x80x94A line in FIG. 16.
In the liquid crystal display panel, a first substrate 1 and a second substrate 5 made of a transparent material such as glass or the like are oppositely disposed to each other, a predetermined gap between them is kept by a spacer not shown, peripheries of the substrates are bonded together by a sealant 4 serving as an adhesive, and a liquid crystal layer 8 is filled in the gap and sealed by an end-sealing material 26.
M pieces of scanning electrodes 2 made of a transparent electrode film are formed on the inner face of the first substrate 1, N pieces of data electrodes 6 intersecting the scanning electrodes 2 are formed on the inner face of the second substrate 5, and intersections of the scanning electrodes 2 and the data electrodes 6 form pixel portions 21 to form a liquid crystal cell of a matrix type liquid crystal display panel having an Mxc3x97N piece of pixel portions.
In FIG. 16, since the second substrate 5 positioned on the uppermost side of this liquid crystal cell is transparent, the data electrodes 6, the first substrate 1 and the scanning electrodes 2, the sealant 4, the end-sealing material 26, and the like positioned thereunder are all shown by solid lines.
In the above liquid crystal display panel, there is a liquid crystal display panel of an active-matrix type having a switching element in each pixel portion 21 and a liquid crystal display panel of a passive-matrix type without providing a switching element, and the liquid crystal display panel as the passive-matrix type is explained here.
It should be noted that, as shown in FIG. 17, an alignment layer 3 and an alignment layer 7 are formed on the inner face of the first substrate 1 and the scanning electrodes 2 and on the inner face of the second substrate 5 and the data electrodes 6 respectively in order to align the liquid crystal molecules of the liquid crystal layer 8 regularly.
Moreover, a first polarizing film 11 is disposed on the rear face side of the first substrate 1, which is the side opposite to the observer""s side (the visible side: the upper side in FIG. 17) of the liquid crystal cell, and a second polarizing film 12 is disposed on the front face side of the second substrate which is the observer""s side.
The first polarizing film 11 is a reflection-type polarizing film and, for example, DBEF (trade name) manufactured by Sumitomo 3M Ltd. is used, and the second polarizing film 12 is an absorption-type polarizing film. On the rear face of the first polarizing film 11, a printed layer 13 of black ink is provided as a light absorbing layer.
The first polarizing film 11 and the second polarizing film 12 are disposed in such a manner that the respective transmission axes are orthogonal to each other, and for the liquid crystal layer 8, used is twisted nematic liquid crystal is used for optically rotating passing light about 90 degrees between the first substrate 1 and the second substrate 5.
Therefore, when the environment is bright where this liquid crystal display panel is used, external light is incident from the front face side of the second substrate 5. Therefore, in a pixel portion for performing a dark display, a first incident ray of light L1 passes through the second polarizing film 12 to become a linearly polarized light, and is optically rotated 90xc2x0 by the liquid crystal layer 8 and is incident to the reflection-type polarizing film which is the first polarizing film 11 as a light linearly polarized in the direction parallel to the transmission axis thereof, so that the light passes through the first polarizing film 11 and is absorbed by the printed layer 13 on the rear face thereof.
Moreover, as for a bright display, a second incident light L2 passes through the second polarizing film 12 to become a linearly polarized light, optical rotatory of the liquid crystal layer 8 is lost by applying a large voltage to the liquid crystal layer 8, the incident light into the liquid crystal layer passes through without being optically rotated and is incident to the reflection-type polarizing film which is the first polarizing film 11 as a light linearly polarized in the direction parallel to the reflection axis thereof, so that the light is reflected by the reflection-type polarizing film 11 to become a strong reflected light L3 and passes through the liquid crystal layer 8 and the second polarizing film 12 to emit to the visible side.
As above, this liquid crystal display panel performs the dark display by a linearly polarized light being incident to the transmission axis of the reflection-type polarizing film and an absorption characteristic of the printed layer 13 and enables the bright display by a reflection characteristic of the reflection-type polarizing film.
However, since the printed layer 13 provided on the first polarizing film 11 which is the reflection-type polarizing film does not have a transmission characteristic, even if an auxiliary light source is disposed on the rear face side of the reflection-type polarizing film 11, light from the auxiliary light source is absorbed by the printed layer 13 having the absorption characteristic and hence light does not reach the visible side.
If a display is performed using the auxiliary light source without providing the printed layer 13, the bright display using incident light by the external light source becomes a dark display in the display by the auxiliary light source, and conversely, the dark display using incident light by the external light source becomes a bright display in the display by the auxiliary light source. Consequently, brightness and darkness be come reversed between in the reflection-type display using the external light source and in the transmission-type display using the auxiliary light source.
As described above, in the case where the auxiliary light source is used in a dark environment, in the conventional liquid crystal display panel in which a light absorbing layer is provided on the rear face of the reflection-type polarizing film, transmittance of light which the auxiliary light source emits is considerably poor, and thus a display can not be performed. Even when the printed layer is removed to apply light of the auxiliary light source to the visible side, a state where a dark display is performed in the reflection-type display becomes a state where the transmittance is large due to the combination of the absorption-type polarizing film and the reflection-type polarizing film, whereby light of the auxiliary light source passes resulting in a bright display, that is, the display in which brightness and darkness thereof is reversed.
Similarly, a state where a bright display is performed in the reflection-type display becomes a state where the transmittance is small because the transmission axis of the absorption-type polarizing film and the transmission axis of the reflection-type polarizing film are orthogonal to each other, whereby light of the auxiliary light source is shut out resulting in a dark display, that is, the display in which brightness and darkness thereof is reversed.
Further, also in the case where a cholesteric liquid crystal polymer is used in place of the reflection-type polarizing film, the display mode thereof becomes nearly the same as in the case where the reflection-type polarizing film is used.
More specifically, the cholesteric liquid crystal polymer selectively and largely reflects (selective reflection) light within a predetermined wavelength region out of the visible light and transmits light within the other wavelength region. Accordingly, the cholesteric liquid crystal polymer has a large reflection characteristic to the external light source within a wavelength region of the selective reflection and a transmission characteristic within the other wavelength region. Therefore, the bright display is performed using the selective reflection of the cholesteric liquid crystal polymer and the dark display is performed by an absorbing material disposed on the rear face side of the cholesteric liquid crystal polymer.
In the case where the auxiliary light source is turned on, light is shut out within the wavelength region of the selective reflection, resulting in a dark display and light of the auxiliary light source is transmitted within the other wavelength region resulting in a bright display. Accordingly, brightness and darkness of the display are reversed.
As described above, in the case of a conventional translucent-type (transflective) liquid crystal display device using the reflection-type polarizing film and having the auxiliary light source, the display comes to one in which the bright display in the reflection display by the external light source and the bright display in the transmission display by turning on the auxiliary light source are reversed. Moreover, since the printed layer provided on the rear face of the reflection-type polarizing film prevents light from the auxiliary light source from transmitting, it is difficult to concurrently use the transmission display by the auxiliary light source.
Also in the case where the cholesteric liquid crystal polymer is used as a reflector, in the transflective liquid crystal display device having the auxiliary light source, the display comes to one in which the bright display in the reflection display and the bright display in the transmission display by turning on the auxiliary light source are reversed. Furthermore, there is a disadvantage that when the printed layer is provided on the rear face of the cholesteric liquid crystal polymer, light by the auxiliary light source can not be transmitted.
The present invention is made to solve the aforesaid disadvantage, and its object is to make it possible to use emitted light of an auxiliary light source efficiently and to prevent brightness and darkness of a display from being reversed between a reflection display by an external light source and a transmission display by use of the auxiliary light source, thereby realizing a display always with high brightness and excellent visibility, in a liquid crystal display device using a reflection-type polarizing film or a cholesteric liquid crystal polymer film.
To achieve the above object, the present invention provides a liquid crystal display device structured as follows.
A liquid crystal display device of the present invention comprises a liquid crystal cell made by disposing a first substrate provided with scanning electrodes and a second substrate provided with data electrodes in such a manner that the scanning electrodes and the data electrodes are oppositely disposed to each other with a predetermined gap therebetween, and a liquid crystal layer being filled between the first substrate and the second substrate.
Moreover, the second substrate of the liquid crystal cell is disposed on the visible side, a polarizing film is disposed on the side of each face of the first substrate and the second substrate opposite to the face contacting the liquid crystal layer, the polarizing film disposed on the first substrate side is a reflection-type polarizing film, an auxiliary light source is provided on the side of the reflection-type polarizing film opposite to the face on the first substrate side, and a light absorbing layer for transmitting part of light is provided between the reflection-type polarizing film and the auxiliary light source.
Furthermore, means for varying a voltage applied to the liquid crystal layer between the auxiliary light source being turned on and being turned off is provided.
It is preferable that the above means is a circuit for reversing a gradation signal applied to the liquid crystal layer between the auxiliary light source being turned on and being turned off.
Moreover, it is preferable to provide a scattering film having light scattering properties between the reflection-type polarizing film and the light absorbing layer.
It is possible to provide the scattering film adhering to the reflection-type polarizing film.
Furthermore, as the scattering film, it is also possible to bond a plastic film in which asperities are formed on the front face thereof or a plastic film in which beads with different refractive indexes are dispersed to the rear face of the reflection-type polarizing film with an adhesive.
It is preferable to provide a gap between the scattering film and the light absorbing layer.
The light absorbing layer can be made of a printed layer having opening portions with a large transmittance and absorbing portions with a small transmittance.
It is desirable that the light absorbing layer has a plurality of opening portions or portions having a transmission characteristic at a pixel portion.
It is preferable that the light absorbing layer having the opening portions has a grid form in which the opening portions and the absorbing portions are arranged regularly and the printed layer forming the absorbing portions is made of a thick film for absorbing external light by the thickness thereof when an incident angle of the external light becomes large.
The light absorbing layer may have a plurality of opening portions having different transmittances, or the light absorbing layer may be made so that the light absorbing layer as a whole transmits part of light. Alternatively, the light absorbing layer may be made of a plurality of absorbing portions having different spectral characteristics within a visible light region.
Alternatively, it is also possible that liquid crystal display device according to the present invention comprises the same liquid crystal cell as above, wherein the second substrate of the liquid crystal cell is disposed on the visible side, an absorption-type polarizing film is disposed on the side of a face of the second substrate opposite to the face contacting the liquid crystal layer and a cholesteric liquid crystal polymer film is disposed on the side of a face of the first substrate opposite to the face contacting the liquid crystal layer respectively, an auxiliary light source is disposed on the side of a face of the cholesteric liquid crystal polymer film opposite to the face on the first substrate side with a light absorbing layer for transmitting part of light therebetween, and means for varying a voltage applied to the liquid crystal layer between while the auxiliary light source is turned on and while it is turned off is provided.
It is preferable that the above means in this case is also a circuit for reversing a gradation signal applied to the liquid crystal layer between while the auxiliary light source is turned on and while it is turned off.
In this liquid crystal display device, it is preferable that a retardation film is disposed between the second substrate and the absorption-type polarizing film, and a xc2xc xcex (quarter-wavelength) film is disposed between the first substrate and the cholesteric liquid crystal polymer film respectively.
It is possible that the light absorbing layer is a printed layer having transparency so that the light absorbing layer as a whole transmits part of light.
Alternatively, the liquid crystal display according to the present invention comprises a liquid crystal cell made by disposing a first substrate provided with scanning electrodes and a second substrate provided with data electrodes in such a manner that the scanning electrodes and the data electrodes are opposed to each other with a predetermined gap therebetween, a liquid crystal layer being filled between the first substrate and the second substrate, and a first color filter being provided on the first substrate or the second substrate, in which the second substrate of the liquid crystal cell is disposed on the visible side.
Moreover, an absorption-type polarizing film is disposed on the side of a face of the second substrate opposite to the face contacting the liquid crystal layer and a reflection-type polarizing film is disposed on the side of a face of the first substrate opposite to the face contacting the liquid crystal layer respectively, an auxiliary light source is disposed on the side of a face of the reflection-type polarizing film opposite to the face on the first substrate side with a second color filter therebetween, and means for varying a voltage applied to the liquid crystal layer between the auxiliary light source being turned on and being turned off is provided.
Moreover, it is preferable to provide a scattering film having light scattering properties between the first substrate and the second color filter.
As the scattering film, it is possible to use a plastic film in which asperities are formed on the front face thereof or a plastic film in which beads with different refractive indexes are dispersed.
It is preferable to use the first color filter and the second color filter respectively made of red, green and blue color filters.
It is preferable that the first color filter and the second color filter are provided on one face side of the first substrate and on the other face side respectively and disposed in such a manner that the color filters of the same color overlap one another with the first substrate therebetween in almost the same areas.
It is also possible to provide means for controlling a voltage applied to the liquid crystal layer of the liquid crystal cell to vary brightness of a display of a halftone in accordance with brightness of an external environment.
The printed layer is provided directly on or across a medium such as a film or the like on the rear face of the reflection-type polarizing film used in the liquid crystal display device of the present invention. The printed layer is provided with the opening portions with a large transmittance and the auxiliary light source is disposed on the rear face of the printed layer having the opening portions, whereby in an environment where the external light source as a main light source is bright (the reflection display), a bright display is performed by the use of the reflective characteristic of the reflection-type polarizing film and a dark display is performed by absorption of the printed layer. In a dark environment (the transmission display), the auxiliary light source is turned on and light of the auxiliary light source is applied to the observer""s side through the opening portions provided in the printed layer to perform a bright display. In this case, the gradation reversal circuit is switched between during ON and during OFF of lighting of the auxiliary light source on the circuit side, so that a small voltage is applied to the liquid crystal layer in the transmission display at a pixel portion where a large voltage is applied to the liquid crystal layer in the reflection display. Conversely, in the reflection display, a large voltage is applied to the liquid crystal layer in the transmission display at a pixel portion where a small voltage is applied to the liquid crystal layer in the reflection display.
By the gradation reversal circuit, in the reflection display and the transmission display, large on small voltages applied to the liquid crystal layer are reversed and simultaneously a reversal of bright and dark displays occurs by the reflection-type polarizing film, so that the bright display in the reflection display becomes the bright display also in the transmission display in the liquid crystal display device. Especially, as for prevention of the reversal of the bright and dark displays of an image, since information of colors is changed between the reflection display and the transmission display due to the reversal of the bright and dark displays in a color liquid crystal display device using a color filter or the like, the prevention of the reversal of the bright and dark displays is very effective.
Similarly to the above, in the case where the cholesteric liquid crystal polymer film is used on the rear face side of the first substrate, the printed layer is provided directly on or across a medium such as a film on the rear face of the cholesteric liquid crystal polymer film, the printed layer is provided with the opening portions with a large transmittance, and the auxiliary light source is disposed on the rear face of the printed layer having the opening portions, whereby in a bright environment (the reflection display), a bright display is performed by use of the reflection characteristic of the reflection-type polarizing film and a dark display is performed by absorption of the printed layer. In a dark environment, (the transmission display), the auxiliary light source is turned on and light of the auxiliary light source is applied to the observer""s side through the opening portions provided in the printed layer to perform a bright display. In this case, the gradation reversal circuit is switched between during ON and during OFF of lighting of the auxiliary light source on the circuit side, so that a small voltage is applied to the liquid crystal layer in the transmission display at a pixel portion where a large voltage is applied to the liquid crystal layer in the reflection display. Conversely, a large voltage is applied to the liquid crystal layer in the transmission display at a pixel portion where a small voltage is applied to the liquid crystal layer in the reflection display.
By the gradation reversal circuit, in the reflection display and the transmission display, large or small voltages applied to the liquid crystal layer are reversed and simultaneously a reversal of bright and dark displays occurs by the cholesteric liquid crystal polymer film, so that the bright display in the reflection display becomes the bright display also in the transmission display in the liquid crystal display device. Especially, as for prevention of the reversal of the bright and dark displays of an image, since information of colors is changed between the reflection display and the transmission display due to the reversal of the bright and dark displays in a color liquid crystal display device using a color filter or the like, the prevention of the reversal of the bright and dark displays is very effective.
Moreover, the first color filter is provided between the first substrate and the second substrate and the second color filter is provided on the rear face of the reflection-type polarizing film or the cholesteric liquid crystal polymer film, whereby in the case of the reflection display, light passes through the first color filter twice to display color information, and in the dark display the second color filter is employed as an absorbing layer by the absorption characteristic thereof. Meanwhile, in the case of the transmission display while the auxiliary light source is turned on, since light passes through the second color filter and the first color filter to display color information, the first color filter can be optimized for the reflection display and chroma of the transmission display can be improved by the second color filter compared with the case where only the first color filter is provided, thereby enabling improvement of both the displays of the reflection display and the transmission display.
It should be noted that since the reflection display requires a bright display, a gradation signal is so set that the halftone thereof is deviated in the bright direction. Thereby, a bright display becomes possible. However, in the case of the transmission display, contrast and chroma are important. Especially, when the external light source is very dark, since an observer becomes very sensitive to brightness, the gradation signal is so set that the halftone thereof is deviated to black side compared with the reflection display, thereby preventing a large change in display by ON and OFF of lighting of the auxiliary light source. In other words, correction of the gradation signal is changed in accordance with ON and OFF of lighting of the auxiliary light source, thereby making the display quality excellent.
Furthermore, correction of the gradation signal by which halftone is deviated in a bright direction or in a dark direction can be changed depending on the brightness of the environment where the liquid crystal display device is used by the external light source, thereby enabling improvement of the display quality also in the case where the reflection display is performed.